This invention relates generally to methods and apparatuses for synchronization and modulation type detection. More particularly, this invention relates to methods and apparatuses for synchronization and modulation type detection in a communication system.
In some cellular communication systems, it can be advantageous to use different modulation types for exchanging information, e.g., between transmitter such as a base station and a receiver such as a mobile station or a fixed cellular terminal. For instance, some modulation types may perform well against interference and background noise at high velocities of the mobile station. This kind of modulation type is normally associated with a low data rate. Other modulation types may provide higher data rates but may be more sensitive to noise, etc. Such modulation types are only suitable at low velocities of the mobile station and in limited areas where the noise is small compared to the received signal strength, e.g., offices or cities.
One example of a cellular system using different modulation types is the proposed EDGE (Enhanced Data Rates for Global System for Mobile Communication (GSM) Evolution) system in which GMSK (the modulation type used in GSM systems today) will be used as the robust low data rate modulation and offset 8-PSK will be used as the modulation for higher data rates.
In communication systems such as these, information is typically exchanged in bursts of a fixed length. Typically, the modulation type is the same within a burst. Using different modulation types in the same digital communication system requires detection of the modulation type used in a particular burst. One simple way to determine the modulation type is to use some kind of signaling between the transmitter and the receiver to indicate which modulation type is being used. However, this signaling decreases the information data rate and may therefore not be desirable. To avoid the need for this signaling, blind modulation detection may be used, requiring detection of the transmitted bits and the modulation type in the receiver. The complexity of the modulation detection device should be kept low in order to minimize the processing power required in the receiver. Additionally, the detection should be adapted to the received signal quality for a good trade-off between complexity and performance.
Various attempts have been made to detect modulation types. For example, U.S. Pat. No. 5,600,673 to Kimura et al. describes the use of phase differences between a recovered data clock and received data for discriminating modulation formats. U.S. Pat. No. 4,933,958 to Brandl et al. describes a similar technique for modulation detection. Neither of these patents addresses the problem of Inter Symbol Interference (ISI) which is common in present radio communication systems, due to multipath propagation of radio waves.
In cellular radio communication systems, it is important that the receiver be synchronized to the transmitter, i.e., the local time reference of the receiver should be synchronized to the time reference of the transmitter. For this purpose, synchronization signals are transmitted periodically by the transmitter. The receiver synchronizes itself with the transmitter, using the synchronization signals.
Therefore, there is a need for an efficient modulation detection technique that, in a simple and reliable way, detects the type of modulation used. Further, the detection unit should be reliable even under severe Inter Symbol Interference (ISI) conditions. To reduce the complexity of the receiver and thus minimize processing power, modulation type detection should be done as soon as possible after receipt of the signal, e.g., at the same time as synchronization.
It is therefore an object of the invention to provide a simple and efficient way for detecting a type of modulation in a received signal. It is a further object of the invention to provide a modulation detection technique that is effective under severe interference conditions. It is yet a further object of the invention to provide a technique for performing or at least attempting modulation type detection simultaneously with synchronization.
According to exemplary embodiments, this and other objects are met by methods and apparatuses for synchronization and modulation detection in a communication system including a transmitter and a receiver. The receiver is synchronized with the transmitter, and a modulation type in a signal transmitted by the transmitter and received by the receiver is detected or an attempt is made to detect the modulation type. A first portion of the received signal is correlated with one or more signals representing modulation types used by the communication system to detect the type of modulation being used in the received signal. Synchronization between the transmitter and the receiver is established.
According to a first embodiment, the first portion of the received signal is correlated with each of one or more different signals representing various modulation types used in the communication system to detect the type of modulation used. According to a second embodiment, the first portion of the received signal is derotated by different amounts to produce a plurality of derotated signals that are correlated with a signal representing the modulation type used by the communication system in order to detect the type of modulation used. According to a third embodiment, if the modulation type is not detected by correlating the first portion of the received signal with each of one or more different signals representing the various modulation types, either a predetermined modulation type is determined to be the modulation type, or the modulation type is decided based, e.g., on a comparison of signal qualities or equalization results. According to a fourth embodiment, if the modulation type is not detected by correlating the derotated signals with a signal representing the modulation used by the communication system, either a predetermined modulation type is determined to be the modulation type, or the modulation type is decided based, e.g., on a comparison of signal qualities or equalization results.